[The] cunning narrative scheme and the peculiarly subtle divulging of the central surprise bring [Emma Tupper's diary] closer to Peter Dickinson's detective stories than to the "Changes" books, Here, it is true, is the same concern for the state of Britain, the same obsession about machines, that have informed all his children's books; man's effect on his environment provides a firm, if implied, moral for an exceptionally exciting story…. Tension and surprise are beautifully managed, in regard to human and extra-human affairs. One can only be thankful for an author who conceives his books for the young on the same grand scale as his adult novels and puts into them ebullient humour and stylishness of expression. (pp. 1749-50)

Margery Fisher, in her Growing Point, May, 1971.